stmhal: Put IRQs into priority order.

- added some comments to explain the priority/sub-priority.
- adds an entry for SDIO (to be used in a later patch)
- increases DMA priority above USB so that DMA can be used
  for sdcard I/O when using USB Mass Storage.

stmhal/irq.h

@@ -39,46 +39,75 @@ MP_DECLARE_CONST_FUN_OBJ(pyb_disable_irq_obj);
 MP_DECLARE_CONST_FUN_OBJ(pyb_enable_irq_obj);
 
 // IRQ priority definitions.
+//
 // Lower number implies higher interrupt priority.
-
-#define IRQ_PRI_CAN 0x7
-#define IRQ_SUBPRI_CAN 0x0
-
-#define IRQ_PRI_DMA 0x6
-#define IRQ_SUBPRI_DMA 0x0
-
-#define IRQ_PRI_EXTINT 0xf
-#define IRQ_SUBPRI_EXTINT 0xf
+//
+// The default priority grouping is set to NVIC_PRIORITYGROUP_4 in the
+// HAL_Init function. This corresponds to 4 bits for the priority field
+// and 0 bits for the sub-priority field (which means that for all intensive
+// purposes that the sub-priorities below are ignored).
+//
+// While a given interrupt is being processed, only higher priority (lower number)
+// interrupts will preempt a given interrupt. If sub-priorities are active
+// then the sub-priority determines the order that pending interrupts of
+// a given priority are executed. This is only meaningful if 2 or more
+// interrupts of the same priority are pending at the same time.
+//
+// The priority of the SysTick timer is determined from the TICK_INT_PRIORITY
+// value which is normally set to 0 in the stm32f4xx_hal_conf.h file.
+//
+// The following interrupts are arranged from highest priority to lowest
+// priority to make it a bit easier to figure out.
+
+//                           Priority   Sub-Priority
+//                           --------   ------------
+//#def  IRQ_PRI_SYSTICK         0
+//#def  IRQ_SUBPRI_SYSTICK                  0
 
 // Flash IRQ must be higher priority than interrupts of all those components
 // that rely on the flash storage.
-#define IRQ_PRI_FLASH 0x1
-#define IRQ_SUBPRI_FLASH 0x1
+#define IRQ_PRI_FLASH           1
+#define IRQ_SUBPRI_FLASH                    1
 
-#define IRQ_PRI_OTG_FS 0x6
-#define IRQ_SUBPRI_OTG_FS 0x0
+#define IRQ_PRI_SDIO            4
+#define IRQ_SUBPRI_SDIO                     0
 
-#define IRQ_PRI_OTG_HS 0x6
-#define IRQ_SUBPRI_OTG_HS 0x0
+// DMA should be higher priority than USB, since USB Mass Storage calls
+// into the sdcard driver which waits for the DMA to complete.
+#define IRQ_PRI_DMA             5
+#define IRQ_SUBPRI_DMA                      0
 
-// PENDSV should be at the lowst priority so that other interrupts complete
-// before exception is raised.
-#define IRQ_PRI_PENDSV 0xf
-#define IRQ_SUBPRI_PENDSV 0xf
+#define IRQ_PRI_OTG_FS          6
+#define IRQ_SUBPRI_OTG_FS                   0
+
+#define IRQ_PRI_OTG_HS          6
+#define IRQ_SUBPRI_OTG_HS                   0
 
-#define IRQ_PRI_RTC_WKUP 0xf
-#define IRQ_SUBPRI_RTC_WKUP 0xf
+#define IRQ_PRI_TIM3            6
+#define IRQ_SUBPRI_TIM3                     0
 
-#define IRQ_PRI_TIM3 0x6
-#define IRQ_SUBPRI_TIM3 0x0
+#define IRQ_PRI_TIM5            6
+#define IRQ_SUBPRI_TIM5                     0
 
-#define IRQ_PRI_TIM5 0x6
-#define IRQ_SUBPRI_TIM5 0x0
+#define IRQ_PRI_CAN             7
+#define IRQ_SUBPRI_CAN                      0
+
+#define IRQ_PRI_UART            13
+#define IRQ_SUBPRI_UART                     13
 
 // Interrupt priority for non-special timers.
-#define IRQ_PRI_TIMX 0xe
-#define IRQ_SUBPRI_TIMX 0xe
+#define IRQ_PRI_TIMX            14
+#define IRQ_SUBPRI_TIMX                     14
+
+#define IRQ_PRI_EXTINT          15
+#define IRQ_SUBPRI_EXTINT                   15
+
+// PENDSV should be at the lowst priority so that other interrupts complete
+// before exception is raised.
+#define IRQ_PRI_PENDSV          15
+#define IRQ_SUBPRI_PENDSV                   15
+
+#define IRQ_PRI_RTC_WKUP        15
+#define IRQ_SUBPRI_RTC_WKUP                 15
 
-#define IRQ_PRI_UART 0xd
-#define IRQ_SUBPRI_UART 0xd